The present invention relates to an image processing method and apparatus for generating image data for printing an image by using print materials (ink) having different densities. The present invention also relates to a printing method and apparatus for printing an image using print materials having different densities.
Printing apparatuses such as printers, copy machines, facsimiles or the like, print an image consisting of dot patterns on a print medium, e.g., paper or a plastic thin plate or the like, based on image data. Such printing apparatuses can be classified by the printing method, e.g., ink-jet type, wire dot type, thermal-transfer type, laser beam type and so on.
Among these types, the ink-jet type (ink-jet printing apparatus) is capable of discharging ink droplets from orifices of a printhead to a print medium.
Recently, a large number of printing apparatuses have been used along with dissemination of computers, and demands are increasing for printing apparatuses which attain high speed printing, high resolution, high quality images, and low noise. Since ink-jet printing apparatuses have a relatively small size, and can readily be applied to color printing, ink-jet printing apparatuses have rapidly become popular.
Many of the ink-jet printing apparatuses employ a printhead where a plurality of ink orifices (nozzles) are integrally arranged, or comprise a plurality of printheads for performing color printing. Furthermore, in order to satisfy the demands for high resolution and high quality image, these ink-jet printing apparatuses utilize half-tone processing method, e.g., dither method or error diffusion method or the like, for faithfully reproducing tones of image data.
An image having excellent tones can be printed by these tone reproduction methods if the resolution of the printing apparatus is sufficiently high (more than 1000 dots per inch). However, in a case where the printing apparatus has a low resolution (about 360 to 720 dots per inch), dots in a highlighted portion become prominently visible, and due to the non-continuity of pixels, a coarse image is produced. In order to increase the number of tones, a method is available for printing a dot having multiple tones. For instance, a voltage or a pulsewidth applied to a printhead is controlled to modulate a print dot diameter which is attached to a print medium, and an image having tones is reproduced by using the dots. However, such method is highly dependent upon its environment. Moreover, print dot diameters tend to fluctuate, and the size of the smallest printable dot is limited. Thus, it is difficult to stably reproduce an image having tones by modulating the diameter of a dot.
Moreover, a density modulation method is available, where dot density in a dot matrix (predetermined area) is changed while keeping the dot size constant. However, since this requires a large image area to increase the number of tones, resolution declines.
As a method of improving tone representation and obtaining an image having high density and high tonality by an ink-jet printing apparatus, a number of printing methods have been proposed and put into practice. One is called a multi-droplet method where a dot is formed by discharging a plurality of ink droplets on substantially the same position of a print medium. By changing the number of ink droplets discharged on a pixel, a pixel having tones is printed. Another method utilizes a plurality of inks having different densities to reproduce tones by combining at least two types of print dots having different densities of the same color group. Alternatively, a printing method combining the above-described two methods is also proposed and put into practice.
It is generally known that gradual evaporation of ink solvent in an ink-jet head unit causes an increase in ink density as the time lapses. As a result, particularly in the multi-droplet method or in the ink-jet printing method employing ink of different densities of the same color group for reproducing tones, the reproduced tones lose their balance due to the ink density fluctuation, causing deterioration of smooth tones in a reproduced image, and sometimes causing deterioration of reproduced image quality by forming a pseudo contour.
For a print medium (print sheet) on which printing is performed, there are more than ten types of printing media having different quality, durability and price depending on the usage purposes. Depending on the combination of a print sheet and ink used, or the combination of ink types superimposed, the ink absorption quantity, ink absorption speed, and the size of generated dot diameters vary. For this reason, sometimes a printed image has a skip of density (the density of the printed image does not correspond to density of the image data), reversed density or the like. Furthermore, in a case of printing with ink having different densities, since the above-described density fluctuation due to evaporation of ink solvent does not occur at an equal rate, a skip of density may result. Due to the foregoing causes, tonality of reproducing density is sometimes so deteriorated that it cannot be corrected by normal calibration.